1. Field of the Invention
The present invention is directed to a conveyor device for conveying a thin web such as a film or photographic paper within a station of a phototype setting machine and the method of conveying.
2. Prior Art
Presently known thin film conveying systems, which are used with phototype setting machines, are generally made in such a way as to enable unrolling a certain amount of film from a supply reel, passing it through an exposing window and then rewinding it on a take up reel after the exposure of the film. In front of the exposure window, the film is guided by upper and lower plates or platens which use the principles that are usually the same as the ones adopted for most cameras. The unwinding or supply reel is supported in such a way as to be able to rotate freely on its axis; however, any axial shift is impossible on account of the arrangement and design of the axle bearings of the unwinding or supply reel. The rewinding or take up reel is equally supported between two bearings and cannot undergo any axial shift. The drive for advancing the web is conceived so that when the leader of the film has been seized by a pair of pinchers or other fastening devices, the film will be wound on the take up reel after each exposure. This type of conveyance device necessitates a perfect alignment of both the supply and take up reels, a constant web width, and adequate web tension as well as step-by-step control of the take up reel drive motor.
Other types of devices use both storage or supply cassettes and take up or rewinding cassettes which are free to rotate on their axis. The web drive is obtained by means of an upper roller operating jointly with a lower roller with one of the two rollers being driven by a stepping motor.
Conveyor devices of these types or kind enable a film to be shifted in both directions, i.e. forward and backward, through an exposure station. Such an operation is necessary on account of the fact that after the exposure of several text lines, it is often useful to be able to step back for the purpose such as paging, structuring of adjacent text columns, or composing a table.
The principle use for the conveyance of film as well as the conveyors described above are likely to impair accurate registration between the various lines or exposed signs. In fact, supposing that a column of thirty lines has been composed on the right hand side of a film and that a second column of the same dimension is to be arranged on the left hand side, it is necessary to pull back the film by a distance corresponding to the length of the first column, shift either the film or the character projection device sideways and to expose the new or second column line-by-line.
In the known devices, an undersirable shift takes place between the line alignment of one of the columns with regard to the other column. Differences due to such shifts are caused by the fact that the film pulled out from the supply cassette is tensioned and hence, stretched during composition of the first column. When the step back or reverse movement of the film for composing the second column occurs, the film is compressed and hence, shortened with regard to the length originally withdrawn from the supply cassette. These two effects do not neutralize each other and, for example, when tables with frames or borders and vertical lines as well as horizontal lines are being composed, the effects will cause imperfections such as unparallel lines, not fully shaped frames or borders, character lines deleted instead of underlined, and other defects. Similar imperfections will also show up if the web slides or slips on the drive means.